A wireless IC tag has identification information about a product or the like to which it is adhered, and by reading the identification information, is used for widespread application such as is read for stock control, product inspection, and history management. For example, the wireless IC tags are used to individually identify and manage a large number of cartridge products housed in a carton. In this case, even when two products are close to each other, to correctly identify them by IC tags, wireless IC tags tolerant of radio wave interference are required.
As an example of wireless IC tags tolerant of radio wave interference, technology disclosed in Japanese Patent Laid-Open No. H11-53492 exists. In this prior art, in ID tags that receive operational power from the outside, when plural ID tags are close to each other, in order that they exert no bad influence to other ID tags during reception of radio waves for power, a resonance circuit comprising a coil for antennas and a resonance capacitor is provided to receive radio waves sent from the outside. A relay is provided in a serial circuit comprising the coil and the resonance capacitor, performs predetermined processing for multi-read commands for external equipments to obtain data from plural ID tags, and then turns off the relay to avoid the reception of radio waves sent from the outside.
As another example of wireless IC tags, technology disclosed in Japanese Patent Laid-Open No. 2000-332523 exists. This technology intends to facilitate manufacturing by broadening the band without deteriorating antenna sensitivity of wireless IC tags and enhancing yield against manufacturing variations. To achieve this, the wireless IC tag includes: a ½-wavelength antenna composed of a microstrip line that includes an antenna, a grounded conductor, and a dielectric between the antenna and the grounded conductor; and one or more wire-shaped conductors for connecting a middle point of the antenna and the grounded conductor, and incorporates an IC between the antenna and the grounded conductor.
The prior art having the antenna composed of a microstrip line has a back side antenna pattern and a surface antenna pattern that are shorted by a through hole, and perform impedance matching by a side short pattern. In the prior art, the shape of the back antenna pattern deciding the antenna size of the wireless IC tag is about 50 to 60 mm in the longer antenna and about 5 to 15 mm in the shorter antenna.
On the other hand, semiconductor chips for a wireless IC tag connected to an antenna that have a size of 0.5 mm or less are disclosed in Japanese Patent Laid-Open Nos. 2002-184872, 2003-296679, 2004-46903, and 2004-78991.